The present invention relates to an output buffer circuit for use in a bidirectional input/output circuit which is, for example, used in an input/output port of a microcomputer or the like.
An integrated circuit, such as a microcomputer, requires a large number of input terminals and a large number of output terminals; however, in order to reduce the number of such terminals, bidirectional input/output circuits are provided. Usually, one bidirectional input/output circuit comprises an output buffer circuit, an input buffer circuit and an input/output terminal to which the output buffer circuit and the input buffer circuit are commonly connected. In this case, an internal output signal generated from the interior of the integrated circuit is transmitted through the output buffer circuit and the input/output terminal to an external load, while an external input signal generated from the exterior of the integrated circuit is transmitted through the input/output terminal and the input buffer circuit to the interior of the integrated circuit.
A first conventional output buffer circuit for use in a bidirectional input/output circuit comprises an inverter circuit. The inverter circuit is formed by a depletion type MOS (broadly, MIS) transistor which serves as a pull-up load and an enhancement type MOS transistor which serves as a driver. In this case, the pull-up resistance of the pull-up transistor is much larger than the on-state resistance of the driver transistor. However, in this output buffer circuit, due to the high resistance of the pull-up transistor and the large external load capacitor there is a disadvantage in that the rising of an external output signal is slow.
A second conventional output buffer circuit for use in a bidirectional input/output circuit comprises an off-buffer circuit instead of the pull-up depletion type transistor of the first conventional output buffer circuit. The off-buffer circuit includes an inverter circuit and a pull-up enhancement type MOS transistor. In this case, it is unnecessary to consider a ratio of the on-state resistances of the two enhancement type transistors. According to the second conventional output buffer circuit, the slow rising of an external output signal is considerably improved; however, since one of the two enhancement type transistors is always turned on, the output buffer circuit is in a low-impedance state from the side of the input/output terminal, which increases the load of the input buffer circuit. Therefore, the second output buffer circuit is not suitable for a bidirectional input/output circuit.
A third conventional output buffer circuit for use in a bidirectional input/output circuit comprises two enhancement type MOS transistors in addition to the second conventional circuit having an off-buffer circuit, so as to improve the low-impedance of the second conventional circuit. However, this third conventional circuit requires a control circuit for controlling the two additional transistors so that the control thereof is complex.